One reason among others:

http://blog.codinghorror.com/the-case-for-case-insensitivity...

Way too ascii-centric. Getting case transformations right in the presence of unicode can be complicated.

Unicode in identifiers is ugly. I never use that.

Helps make code look more like the algorithm it's implementing - would rather just use α than keep spelling out "alpha" every time. Good editors and REPLs make it easy to enter unicode symbols by tab-completing the latex spelling, e.g. \alpha<tab> gets replaced by α.

I don't use nim, so I couldn't point you to the justification. But as I understand it is so an individual can choose his style preferences. myVariable or my_variable.

here:

http://nim-lang.org/docs/manual.html#lexical-analysis-identi...